1. Field of the Invention
The invention provides a process for producing polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process). This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemo-mechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is required to an increased degree for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for manufacture of solar cells for photovoltaics.
The polycrystalline silicon is typically produced by means of the Siemens process. In this process, in a bell jar-shaped reactor (“Siemens reactor”), thin filament rods (“thin rods”) of silicon are heated by direct passage of current and a reaction gas containing a silicon-containing component and hydrogen is introduced.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X═Cl, Br, I). It is preferably a chlorosilane or chlorosilane mixture, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
EP 2 077 252 A2 describes the typical setup of a reactor type used in the production of polysilicon.
The reactor base is provided with electrodes that accommodate the thin rods on which silicon is deposited during the growth process, and which thus grow to form the desired rods of polysilicon. Typically, two thin rods in each case are joined by a bridge to form a pair of thin rods, which form a circuit through the electrodes and through external devices, which serves to heat the rod pairs to a particular temperature.
Moreover, the reactor base is additionally provided with nozzles that supply the reactor with fresh gas. The offgas is conducted back out of the reaction space via orifices.
The amount of reaction gases supplied is typically varied as a function of the rod diameter, i.e. is generally increased with increasing rod diameter.
High-purity polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD=chemical vapor deposition/gas phase deposition).
DE 102 007 047 210 A1 discloses a process that leads to polysilicon rods having advantageous flexural strength. Moreover, the specific energy consumption in this process is particularly low. With regard to process parameters, a maximum value of the flow rate of the chlorosilanes mixture is attained within fewer than 30 hours, preferably within fewer than 5 hours, with the temperature on the underside of the bridge between 1300° C. and 1413° C.
DE 10 2007 023 041 A1 describes a further process for producing polysilicon, specifically for FZ (float zone) silicon. It envisages a rod temperature of 950 to 1090° C. and a particular proportion of chlorosilanes in the reaction gas up to a rod diameter of 30 mm, and a switch in the rod temperature to 930 to 1030° C. and an increase in the proportion of chlorosilanes in the reaction gas no later than attainment of a rod diameter of 120 mm. Abrupt changes in the growth conditions must not be made over the entire deposition period.
US 20120048178 A1 discloses a process for producing polycrystalline silicon, comprising introduction of a reaction gas comprising a silicon-containing component and hydrogen by means of one or more nozzles into a reactor comprising at least one heated filament rod on which silicon is deposited, wherein an Archimedes number Arn, which describes flow conditions in the reactor as a function of the fill level FL which states the ratio of a rod volume to an empty reactor volume in percent, for a fill level FL of up to 5%, is within a range limited at a lower end by a function Ar=2000×FL−0.6 and at an upper end by a function Ar=17000×FL−0.9, and at a fill level of greater than 5% is within a range from at least 750 to at most 4000.
The fill level of a reactor states the ratio of the volume of the rods to the empty volume of the reactor in percent. The empty volume of the reactor is constant. The fill level thus increases with increasing process duration since the volume of the rods increases.
The Archimedes number is given byAr=π*g*L3*Ad*(Trod−Twall)/(2*Q2*(Trod+Twall))
where g is the acceleration due to gravity in m/s2, L is the rod length of the filament rods in m, Q is the volume flow of the gas in m3/s under operating conditions (p, T), Ad is the sum total of all the nozzle cross-sectional areas in m2, Trod is the rod temperature in K and Twall is the wall temperature in K. The rod temperature is preferably 1150K to 1600K. The wall temperature is preferably 300K to 700K.
It is a relatively common observation in the production of thick polycrystalline silicon rods (having diameter>100 mm) that the rods have regions with a very rough surface (“popcorn”). These rough regions have to be separated from the rest of the material and sold at much lower prices than the rest of the silicon rod.
U.S. Pat. No. 5,904,981 A discloses that a temporary reduction in the temperature of the rods can reduce the proportion of the popcorn material. At the same time, it is disclosed that, proceeding from a polycrystalline silicon rod having a diameter of 5 mm as a filament (thin rod), a surface temperature of the rod is kept at 1030° C. and polycrystalline silicon is deposited, and, when the rod diameter reaches 85 mm, the electrical current is kept constant, as a result of which the temperature falls, and, as soon as a temperature of 970° C. is attained, the temperature of the rods is increased gradually back up to 1030° C. over a period of 30 hours, stopping the deposition when the rod diameter reaches 120 mm. The proportion of popcorn in this case is 13%. The effect of such changes, however, is that the process runs less quickly and hence the output is reduced, which reduces the economic viability.
In the known processes for deposition of polycrystalline silicon, it is thus necessary to regulate the rod temperature. The temperature at the surface of the rods is the crucial parameter in the process for producing polycrystalline silicon, since the polycrystalline silicon is deposited at the rod surface. For this purpose, the rod temperature has to be measured. The rod temperature is typically measured with radiation pyrometers on the surfaces of the vertical rods.
Because of its material properties, contactless temperature measurement on silicon is very demanding. This is because the emission level of the material varies significantly over the infrared spectrum and is additionally dependent on the material temperature. In order nevertheless to achieve exact and repeatable measurement results, the manufacturers provide the instruments with filters to about 0.9 μm, and so evaluate only a small portion of the radiation spectrum, restricted to a particular wavelength range by a filter, since the emission level of silicon within this wavelength range is both relatively high and independent of temperature.
Because of hydrogen in the atmosphere, specific explosion-proof housings are typically used for the pyrometers.
The pyrometer gains optical access through a sightglass or a window. The lens or the window for instruments in the near infrared range consists of glass or quartz glass.
The pyrometers are mounted at the sightglasses outside the reactor and are directed at the polysilicon rod to be measured. The sightglass seals the reactor off from the environment by means of a transparent glass surface and seals.
It has now been found that, in the course of the deposition process, a layer of deposits forms on the sightglass, which may be of different thickness according to the mode of operation. This particularly affects the (inner) glass surface at the reactor end. This layer of deposits causes an attenuation of the measured radiation intensity. As a result, the pyrometer measures temperatures that are too low. The result of this is that the rod temperatures are set too high by the electrical power regulation system of the reactor, which causes unwanted process properties such as dust deposition, impermissibly high popcorn growth, local melting of the silicon rods, etc. In the worst case—namely in the case of excessively thick deposits—the process has to be ended prematurely.
Economic disadvantages as a result of off-spec and hence reduced-value products or increased production costs as a result of prematurely shut-down or failed batches are the consequences of deposits on the sightglass.
In the prior art, efforts have been made to minimize formation of deposits on the glass surfaces, by blowing an inert gas or hydrogen over the glass surface, in order to flush silanes or chlorosilanes, which have a tendency to form deposits on the glass, away from the glass surface, or keep them away from the glass surface.
JP2010254561 A2 describes a sightglass where hydrogen is used as purge gas and is injected into the tube. The ratio of tube length to tube diameter (L/D) in this arrangement is between 5 and 10. A disadvantage is the greatly restricted viewing range resulting from the long, thin sightglass tube.
CN 201302372Y likewise discloses a sightglass where particles adhering on the sightglass lens are to be removed by blowing in gas medium (hydrogen) involved in the reaction, which cleans the lens. The inner connecting tube is connected at one end to a gas medium cleaning apparatus, such that the inside surface of the sightglass lens can be cleaned in the course of operation. Between the first sightglass lens and the second sightglass lens is a cooling water duct, by means of which the first sightglass lens and the second sightglass lens can be cooled and cleaned.
CN102311120 B discloses a sightglass where hydrogen as purge gas is injected through a multitude of holes at an oblique angle to the sightglass surface. The holes are distributed over the entire circumference of the sightglass tube and aligned radially with respect to the axis of the sightglass tube.
However, it has been found that this prevents the formation of deposits only in some regions of the sightglass, but actually enhances it in other regions. Moreover, it has been observed at times that the positions of deposit-free regions on the sightglass surface changes during the process. Thus, reproducible temperature measurements are impossible.
This problem gave rise to the objective of the invention. The sightglass is to remain free of deposits and impurities over the entire batch run.